Demands for ever-greater communication data rates in data center networks have led to increasing use of optical interconnects, and even so have pushed transmission components to their physical limits. These optical interconnects use low-cost, power-efficient optoelectronic components, such as vertical-cavity surface-emitting lasers (VCSELs) and integrated Mach-Zender modulators (MZMs). In order to overcome bandwidth limitations, many links use high-order modulation formats, which in turn require large amplitude swings in the output signals from the transmitter. As a consequence, the transmission components are forced to operate outside their linear regimes. The resulting nonlinear distortion leads to a significant reduction in overall signal/noise ratio (SNR).
Methods for estimating and compensating for nonlinear distortion are known in the art, but they are difficult to implement in practice. Whereas the length of a linear filter tends to grow linearly with the severity of the inter-symbol interference (ISI) for which it is required to compensate, nonlinear filters grow at polynomial rates or even faster, as both the number of non-linear orders and the length of each order increase. (The “length” of a filter, as used in the present description and in the claims, refers to the number on taps in a discrete time-domain implementation of the filter, i.e., the number of input samples that are concurrently multiplied by respective tap coefficients and added to produce each filtered output sample. The filter length is also referred to as the memory depth or memory order.) Therefore, it is often difficult for the designer to satisfy the conflicting demands of both reducing nonlinear distortion and minimizing circuit size, cost and power consumption.